The invention relates to dropout compensation and particularly to dropout compensation performed at the read side of a time base corrector memory, wherein dropout timing is preserved by selectively encoding the incoming video data.
In the field of color television, the process commonly known as dropout compensation is performed during the reproduction of video signals. The corresponding dropout compensators provide compensation by replacing an incorrect or missing portion of the color television signal, commonly called a "dropout", which is caused by an improper recording or reproduction process, or by a defect in the recording medium. Thus, disturbing effects which would be caused in a television picture by the presence of such a dropout, and which are readily seen by a viewer, are removed by the dropout compensation process. Typical of such process and apparatus for performing digital dropout compensation is the system described, for example, in U.S. Pat. No. 4,251,831 to B. Y. Kamath, assigned to the same assignee as this application, wherein a suitable band-pass filter, and a dropout compensator employing the filter, provide digital circuits for separating the components of a composite signal for further processing, and for subsequently recombining the components to reconstitute an altered composite signal during reproduction from magnetic tape.
An example of another prior art digital dropout compensator is described in the manual "AVR-2 Video Tape Recorder, Theory of Operation", Catalog No. 1809179-01, published by Ampex Corporation, November 1977, pages 9-10, 9-14, 9-20, and 9-77 to 9-92.
In order to perform the dropout compensation process, the location of the dropout must be detected and preserved; that is, dropout timing must be preserved to allow subsequently replacing the specific incorrect video data with dropout corrected video data. Since dropout timing inherently is related to tape input video, dropout compensators traditionally have been placed on the input, i.e., write side, of the time base corrector memory employed in a conventional video processing system. Since the input clock of a video signal being read from tape is highly unstable and capable of varying rates of, for example, from 0.5 to 1.7 times normal, as when operating in the high speed shuttle mode, complex counters and very high speed memory are required in a dropout compensator located at the write side of memory in order to perform the dropout compensation process.
More particularly, when the dropout compensator is at the write side of memory it is more dependent upon tape speed. That is, the voltage controlled oscillator located at the input to memory, produces a clock that is continously changing with tape speed as it trys to track the video information off tape. It follows that the dropout compensator circuit must also operate with the unstable clock, particularly when in the shuttle mode of operation. The dropout compensator circuit must be capable of operating over a wide range of frequencies corresponding to the range of tape speeds and the resulting unstable high rate input clock. Thus the requirement for complex counters and the very high speed memory of previous mention.
For the above reasons, it would be preferable to locate the dropout compensator system on the output or read side of memory, since the read clock for the memory always operates at a fixed frequency; for example, at 14.3 megaHertz, (MHz) for a color television recorder system employing a 4Fsc data sample rate. Since a stable clock of fixed frequency is available at the read side of the memory, a dropout compensator of simplified design, with reduced memory speed requirements, may be utilized. However, since the read and write operations, and thus the timing thereof, are operating independently of each other, preserving the location of the dropout on the write side of memory and transferring the timing information to the read side of memory, becomes a problem. The problem is compounded by the fact that time base correction systems typically read out the stored digital video several lines later relative to the location in which the data was loaded into memory. Thus the initial dropout location, that is, the delayed timing information indicative thereof, must be transferred with precision to the output side of memory.
Time base corrector systems are available wherein the dropout compensator is disposed at the read side of memory. Typically in such systems, the dropout timing information is transferred from the write to the read side of memory by employing a separate dropout memory and memory control to preserve and transfer the dropout timing information. To this end, while video data is being written into the TBC memory, the presence or absence of dropouts simultaneously is loaded into the dropout memory. During the readout of video data from the TBC memory, the dropout timing information is simultaneously extracted from the dropout memory, whereby the time of occurrence of dropouts in the video data is detected, and dropout compensation is effected. It may be seen that in such systems the location of the dropout compensator at the read side of memory requires an extra memory and associated control circuit in order to preserve the timing information.
Accordingly, it is an object of the invention to locate a dropout compensator system at the read side of a time base corrector memory.
Another object is to circumvent the use of a separate dropout memory and control therefor, while precisely transferring the dropout timing information to the read side of memory.
Still another object is to operate a dropout compensator with the stable read clock of fixed frequency on the output side of memory.
A further object is to provide transfer of the dropout information to the read side of memory by modifing select video data samples to preserve the time of occurrence of dropouts.
A yet further object is to transfer the dropout timing information, in an 8-bit system, by assigning an incorrect video data sample the maximum or minimum digital value of 255 or zero, respectively, and inserting the sample in the video data path.
Still another object is to represent the occurrence of a dropout as a digital word, within the video data path, of all "1"s or all "0"s, whereupon the word is subsequently detected by sensing the all "1"s or all "0"s.
The foregoing and other objects are accomplished by the invention while overcoming the various disadvantages of the above-mentioned prior art, by locating the dropout compensator system at the read side of the TBC memory, while selectively encoding the video data as it is being written into memory to indicate the exact location of a dropout, or dropouts. To this end, when a dropout occurs as conventionally indicated, for example, by an RF envelope detector in the video tape recorder system, the video samples are modified to provide a digital value which is indicative of the occurrence of the dropout. For example, in an 8-bit digital system, the invention comtemplates the use of an 8-bit data sample with the maximum digital value of 255 corresponding to all "1" bits (FFH in the hex notation) to indicate that a dropout has occurred. Thus, valid video data is precluded from using the value of 255, but is limited to a maximum digital value of 254, i.e., FEH in the hex notation. The value 255 (FFH) indicative of the dropout is inserted in the video data path along with the valid video data. During read out of the video data from the TBC memory, the value 255 readily is detected on the read side of memory and is used to activate the associated dropout compensator circuit to replace the incorrect video data with dropout compensated video data.